The present invention relates to a magnetic resonance imaging apparatus, more specifically, a gradient coil apparatus for producing a gradient magnetic field whose intensity of the magnetic field spatially changes.
A magnetic resonance imaging method is a method in which internal data of a subject can be obtained from an outer section. This magnetic resonance imaging method is known well. More specifically, a high frequency magnetic field, which rotates at a specific frequency, is applied to an atomic unclear, which is positioned in an even static magnetic field, that is, a magnetizing spin. As a result, the magnetizing spin absorbs energy. If the high frequency magnetic field is turned off, the magnetizing spin discharges the absorbed energy. Such absorbing and discharging phenomenon is a magnetic resonance phenomenon.
In such a magnetic resonance imaging method, the tissue structure of the interpolation of the subject can be imaged with high contrast. Moreover, the flow of the blood and the brain function can be imaged.
The brain imaging is a method in which a visual sense and an auditory sense are stimulated by flush light and sound so that the reaction of the brain to the stimulation can be obtained. For example, there is a BOLD (Blood Oxygenation Level Dependent) method. According to the BOLD method, only an active area, which reacts to the stimulation, can be extracted by differentiating images, one is obtained in giving stimulation to the brain, and the other is obtained in giving no stimulation to the brain.
Moreover, in the brain function imaging method, an EPI (Echo Planar Imaging) method is useful. According to the EPI method, data, which is necessary for reconstructing one image, can be collected for an extremely short period of time such as 20 msec. The short time collection of data is repeated. Thereby, it is possible to capture the state that the brain is gradually activated by stimulation, and gradually calmed down. Moreover, in EPI, since the number of signal errors, which is caused by the blood flow, is small, there is an advantage in which the reaction of the brain to the stimulation can be provided with high accuracy.
However, in EPI, it is needed that the gradient magnetic field having high intensity of 20mT/m! or more be alternated at high speed for a short period of time such as 0.1msec!. Such a high speed alternation causes occurrence of an eddy current in the magnetic member such as a radiant shield of superconductive magnet and a helium container. An eddy magnetic field, which is caused by the eddy current, is generated in a direction opposite to the gradient magnetic field. Due to this, since the waveform of the gradient magnetic field is distorted, a phase error occurs in an MR signal, and an artifact is generated on the image, which is finally reconstructed.
To solve the above problem, an active shield type gradient coil (ASGC) is needed in EPI. The active shield type gradient coil is used to cancel the magnetic field leaked from the main coil for generating the gradient magnetic field in the magnetic field generated from the shield coil. As a result, the eddy magnetic field is prevented from being generated.
As mentioned above, since the gradient magnetic field having high intensity of 20 mT/m! must be alternated for the short period of time such as 0.1 mT/m!, the load of the power supply is extremely large. To make the inductance of the coil smaller to reduce the load, an apparatus having a gradient coil whose size is reduced has been developed to be dedicated to the local portion such as a head.
In the conventional brain function imaging, the subject was simply stimulated by flush light. However, in the near future, it can be considered that an optical image is mainly shown to the subject to observe the complicated reaction. Conventionally, as shown in FIG. 1, an optical image from a display generator (not shown) of an outer portion was reflected by a mirror 81 to be transmitted to a subject 106 through a window 95 formed on a gradient coil apparatus 91.
However, in ASGC, the above-mentioned image transmitting method cannot be used. Specifically, the window must be formed on not only the main coil but also a shield coil to use the conventional image transmitting method in ASGC. Due to this, the magnetic field is leaked from the window of the shield coil, and an original shield effect of the magnetic field of ASGC is considerably reduced. This problem may be solved by arranging an optical path and a mirror between the main coil and the shield coil. However, if such an arrangement is provided, the gradient coil apparatus is enlarged.
As mentioned above, in the conventional case, it was impossible to stimulate the brain of the subject by the optical image as using ASGC in the gradient coil apparatus.